Nature and combustion of the coke deposited over a Ni-HZSM-5 catalyst in the transformation of methanol/bio-oil mixtures into hydrocarbons
Sustainable process-product development & green chemistry
Sustainable & Clean Technologies - III: Combustion & Emission (T1-6)
Keywords: bio-oil valorisation, Ni-HZSM-5 zeolite, combustion
The liquid (bio-oil) obtained by fast pyrolysis of vegetable biomass, is considered an interesting alternative to conventional fuels, but its instability and composition (oxygenated nature) make it advisable to carry out catalytic reforming upon acidic catalysts, prior to its combustion in turbines or combustors [1]. A Ni-HZSM-5 zeolite catalyst is suitable for this purpose, because it is hydrothermally stable at high temperatures (up to 500 ºC) in presence of high water content in the reaction medium [2], which is a requirement in order to attain high conversion of bio-oil for this process. Besides, a stabilization treatment of bio-oil by dissolution in methanol is necessary, prior to its catalytic transformation, in order to minimize the plugging problems caused by thermal degradation of lignocellulosic components, especially in the reactor preheating zone. Nevertheless, the catalyst suffers from a rapid deactivation by coke deposition, and must be regenerated by coke combustion with air.
In this paper, the combustion kinetics of the coke deposited over a Ni-HZSM-5 catalyst has been studied in the transformation of bio-oil in a fluidized bed reactor. The bio-oil has been previously stabilized by dissolving it in methanol at different proportions. It has been proven that dissolution of bio-oil in methanol favours its catalytic transformation, by minimizing the problems of reactor plugging in the feed preheating zone [3].
Coke combustion with air has been carried out in a thermobalance (SDT 2960 TA Instruments), by following a temperature ramp ( 3 ºC min -1) from 300 to 550 ºC. From the DTG (mass variation) curves, two combustion peaks have been observed: one corresponding to “thermal” coke (which burns at lower temperature and is associated to the thermal degradation of bio-oil components) and the other corresponding to catalytic coke (formed by coke precursors evolution in the reaction of hydrocarbon formation). In order to individually quantify and to determine the combustion kinetic of both types of coke, a deconvolution program (Matlab) of the DTG curves has been developed, which assumes for both cokes a first order kinetics with respect to oxygen and to coke content. The activation energy corresponding to the combustion of thermal coke (which deposits presumably externally to the crystals of zeolite) is considerably higher than that corresponding to catalytic coke (which is located internally to the zeolite crystals).
It has been proven that as the content of MeOH in the feed is increased, the coke content in the catalyst is considerably lower, mainly the “thermal” coke fraction. Minimizing coke deposition extends considerably the catalyst life, which is an important requirement for the industrial implementation of bio-oil valorisation processes by catalytic transformation.
[1] A.V. Bridgwater, Chem Eng. J. 1992, 4056, 1.
[2] Valle, B., Alonso, A., Atutxa, A., Gayubo, A.G., Bilbao, J. Catal. Today, 2005,106, 118-122.
[3] Valle, B., Alonso, A., Atutxa, A., Gayubo, A.G., Bilbao, J. in Procceding of the 17th International Congress of Chemical and Process Engineering, Praga (Check Republic), 2006, Summaries 1, P7.10, p. 121.
Presented Tuesday 18, 11:20 to 11:40, in session Sustainable & Clean Technologies - III: Combustion & Emission (T1-6).
